【作者】 潘晓亮；

【导师】 甄良； 徐成彦；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 虽然锂离子电池的研发及应用已取得了很大的成功，但是随着新能源的开发与利用，便携式电子产品的飞速发展以及环境保护所面临不断增大的压力，人们对高性能电池的需求也更加迫切，这也为锂离子电池的进一步发展带来了新的机遇与挑战。正极材料是锂离子电池重要的组成部分，它制约着电池性能的进一步发挥以及电池成本的进一步降低。LiMnPO₄是一种在电化学性能和产业化成本上都非常有前途的锂离子电池用正极材料。然而，LiMnPO₄晶体具有很低的电导率，它严重制约着LiMnPO₄电化学性能的发挥。影响LiMnPO₄电导率的因素主要包括晶体的尺寸、分散程度、晶面生长取向以及材料的改性。通过对这些影响因素进行调控，可以改善LiMnPO₄正极材料的电导率，进而提高LiMnPO₄正极材料的电化学性能。本文通过水/溶剂热法，首次采用Na2S·9H₂O作为添加剂，可控地合成出了多种新型形貌的LiMnPO₄微纳米晶，分析了这些新型形貌的形成机制，探究了晶体的尺寸、分散程度、晶面生长取向、掺杂及碳复合对LiMnPO₄正极材料电化学行为的影响规律。通过粉末X射线衍射仪、扫描电子显微镜、透射电子显微镜对产物的物相、结构、形貌进行了精细表征，利用充放电测试、循环伏安测试、电化学阻抗谱测试对LiMnPO₄正极材料的电化学行为进行了详细表征。本文的主要研究成果概括如下。通过水热工艺合成出了多种新型形貌的LiMnPO₄样品，并实现了LiMnPO₄晶粒分散程度及结晶取向的精确可控。这些新型形貌包括分散良好的片状晶、楔形晶、柱状晶以及由相应颗粒自组装形成的微球。三种分散良好的样品具有相近的晶粒分散程度及近似的晶粒尺寸，不同的形状特征及结晶取向；三种微球样品具有近似的球形尺寸，而组装单元的晶体形状特征及结晶取向均不同。分散片状晶及相应微球的组装片状晶暴露有大量的（010）晶面、沿[010]晶向晶体尺寸小的结构特点。三种分散LiMnPO₄样品的电化学性能均要好于相应微球的LiMnPO₄样品，原因主要归结为分散样品的分散形貌优势。片状晶的电化学性能要好于楔形晶和柱状晶，原因主要归结为片状晶的特定晶体学取向优势。通过水热工艺合成出长为1-4μm、宽约500nm、厚约200nm、分散均匀的矩形长棒样品，通过结构表征得到样品的厚度方向为[010]晶向；经过溶剂热反应制备出长为200-600nm、宽约400nm、厚约200nm、分散均匀的短棒样品，以及长为200-400nm、宽约200nm、厚约100nm、分散均匀的短棒样品；矩形长棒样品及短棒样品的晶粒具有近似的形状特征、分散程度，不同的尺寸大小。电化学性能测试结果表明，随着晶粒尺寸的减小，电池的容量性能、倍率性能提高，而电池的循环稳定性能略有下降。水/溶剂热产物的形貌是在劈裂机制作用下得到的。产物的物相是先得到中间相NH4MnPO₄·H₂O及MnHPO₄·2.25H₂O，然后生成LiMnPO₄相。产物的晶体生长取向可能受乙酸根离子或/和硫离子或/和氢氧根离子的作用。掺杂样品LiMn_0.95M_0.05PO₄（M=Al, Ce, Fe, In）以及LiMn_1-yFe_yPO₄（y=0.1,0.3,0.5）的电化学性能均较未掺杂样品的电化学性能差，主要原因可能是由于掺杂离子掺杂到Li位导致锂离子扩散通道堵塞所造成的。通过蔗糖原位复合及非原位复合获得的LiMnPO₄/C样品较Super P球磨复合后所获得的LiMnPO₄/C样品的倍率性能、循环稳定性能要好，而在低倍率下容量性能略有下降。例如，3g蔗糖原位复合所得的样品及Super P球磨复合所得的样品，在室温0.05C、0.1C、0.2C、0.5C及1C充放电倍率下，分别获得了139.4、132.6、129.1、119.9、109.7mA h g^-1，以及142.1、126.1、98.1、81.5、70.5mAh g^-1的放电比容量；在室温、0.1C充放电倍率、2.4-4.5V电压范围的条件下，50次充放电循环后它们的循环效率分别为93.7%及91.4%。更多还原

【Abstract】 Along with the development and utilization of new energy sources, the rapiddevelopment of portable electronic products, and the increasing pressure on facingenviromental protection, the demand for lithium ion batteries with highperformances is more urgent than ever, which brings new opportunities andchallenges to further development for lithium ion batteries, yet the researches andapplications of lithium ion batteries have achieved great successes. The cathodematerial is an important part of lithium ion battery, which restrict the performancesfor the further improvement and the cost for the further reduction. LiMnPO₄is avery promising cathode material in electrochemical performance and industrial costfor lithium ion battery. However, LiMnPO₄suffers from very low electricalconductivity, which leads to poor electrochemical properties. The main factorsgoverning the electrical conductivity of LiMnPO₄probably include grain size,dispensability, cryallographic orientation and modification, which can bemanipulated for improving the electrical conductivity and enhancing theelectrochemical properties of LiMnPO₄.Several novel morphologies of LiMnPO₄micro-/nanocrystals had beencontrollably synthesized by employing Na2S·9H₂O as a sole additive viahydrothermal/solovthermal routes. The possible formation was proposed. Theeffects of grain size, dispensability, crystallographic orientation, and modificationon electrochemical behaviors of LiMnPO₄cathode material were discussed. Theobtained phases, structures and morphologies of LiMnPO₄samples werecharacterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）,transmission electron microscopy （TEM）. The charge/discharge measurement, cyclicvoltammetry （CV） measurement, and electrochemical impedance spectroscopy （EIS）measurement were carried out to test the electrochemical behavior of LiMnPO₄cathode material. In this work, we have obtained the following conclusions.Several novel LiMnPO₄morphologies had been controllably synthesized byhydrothermal method, including microspheres assembled by plates, wedges, prisms,and disperse morphologies with plates, wedges, prisms, respectively. Thecrystallographic orientations and dispensabilities of LiMnPO₄crystals werecontrollably manipulated. The three dispersed morphologies samples had a similardegree of the dispensabilty as well as size of the grain, and different the shapefeatures and the crystallographic orientations. The three microspheres morphologiessamples possessed a similar size of microsphere, yet the assembly units of themicrospheres had different the shape features and the crystallographic orientations. The dispersed the plates and the microsphere assembled with plates possessed largepercentage of exposed （010） facets as well as small thickness along the [010]direction. The three dispersed morphologies samples exhibited betterelectrochemical properties than the corresponding microspheres samples. Theexcellent electrochemical performance of the dispersed samples can be attributed toits well-dispsered morphology advangtage. The plate-like crystals displayedsuperior electrochemical performance over the wedge-like, and prism-like crystals,which can be attributed to its special crystallographic orientation.The well-dispersed rectangular rods with a length of1-4μm, a width of⁵00nm and a thickness of around200nm were prepared by hydrothermal process. The[010] direction was just along the thinnest part of the rectangular rods. Thewell-dispersed rods with a length of200-600nm, a mean width of ca.500nm andthickness of around200nm, and the well-dispersed rods with a length of200-400nm, a width of ca.200nm and a thickness of around100nm were obtained bysolvothermal route. These rods had a similar shape feature as well as dispensability,and different the grain sizes. The results of the electrochemical measurementsconfirmed that the charge/discharge and the rate capability of the cells wereenhanced, and the cycling stability of the cells was slightly decreased along with thereduction of the grain sizes.The splitting process was proposed to elucidate the growth mechanism of themorphologies synthesized by hydrothermal and solvothermal methods. Themesophases of NH4MnPO₄·H₂O and MnHPO₄·2.25H₂O were formed at the earlyreaction stage, and finally the LiMnPO₄phase was obtained. The crystallographicorientations of the as-prepared crystals could be strongly governed by acetate ionand/or sulphion and/or hydroxyl.The electrochemical properties of doping samples such as LiMn_0.95M_0.05PO₄（M=Al, Ce, Fe, In） and LiMn_1-yFe_yPO₄（y=0.1,0.3,0.5） were worse than that of thewell-dispersed rods, which could be ascribe to the blocking of the diffusion channelfor lithium ion owing to the doping cation in the lithium ion site. The ratecapabilities and cycling stabilities of the LiMnPO₄/C composites obtain by in-situand ex-situ routes with sucrose were better than that of the LiMnPO₄/C compositesprepared by ball milling method with Super P, yet the capacities of the LiMnPO₄/Ccomposites obtained by sucrose slightly decreased at low charging/discharging rate.For example, The LiMnPO₄/C composites prepared by in-situ route with3g sucroseexhibited the discharge capacities of139.4,132.6,129.1,119.9and109.7mA h g^-1at0.05C,0.1C,0.2C,0.5C and1C, respectively, whereas the LiMnPO₄/Ccomposites obtained by ball milling method with Super P delivered the dischargecapacities of142.1,126.1,98.1,81.5and70.5mA h g^-1at0.05C,0.1C,0.2C,0.5C and1C, respectively.93.7%and91.4%of the initial discharge capacities could be retained over50cycles at a charging/discharging rate of0.1C at roomtemperature in cell potential range of2.4-4.5V for the LiMnPO₄/C compositesobtained by3g sucrose and Super P, respectively.更多还原